CONGESTION CONTROL
Congestion Control <ul><li>When one part of the subnet (e.g. one or more routers in an area) becomes overloaded, congestio...
Factors that Cause Congestion <ul><li>Packet arrival rate exceeds the outgoing link capacity. </li></ul><ul><li>Insufficie...
Congestion Control vs Flow Control <ul><li>Congestion control is a global issue – involves every router and host within th...
Congestion Control, cont. <ul><li>Congestion Control is concerned with efficiently using a network at high load. </li></ul...
Warning Bit <ul><li>A special bit in the packet header is set by the router to warn the source when congestion is detected...
Choke Packets <ul><li>A more direct way of telling the source to slow down. </li></ul><ul><li>A choke packet is a control ...
Hop-by-Hop Choke Packets <ul><li>Over long distances or at high speeds choke packets are not very effective.  </li></ul><u...
Load Shedding <ul><li>When buffers become full, routers simply discard packets. </li></ul><ul><li>Which packet is chosen t...
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Congestion Control

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Unit 4 Of ACN

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Congestion Control

  1. 1. CONGESTION CONTROL
  2. 2. Congestion Control <ul><li>When one part of the subnet (e.g. one or more routers in an area) becomes overloaded, congestion results. </li></ul><ul><li>Because routers are receiving packets faster than they can forward them, one of two things must happen: </li></ul><ul><ul><li>The subnet must prevent additional packets from entering the congested region until those already present can be processed. </li></ul></ul><ul><ul><li>The congested routers can discard queued packets to make room for those that are arriving. </li></ul></ul>
  3. 3. Factors that Cause Congestion <ul><li>Packet arrival rate exceeds the outgoing link capacity. </li></ul><ul><li>Insufficient memory to store arriving packets </li></ul><ul><li>Bursty traffic </li></ul><ul><li>Slow processor </li></ul>
  4. 4. Congestion Control vs Flow Control <ul><li>Congestion control is a global issue – involves every router and host within the subnet </li></ul><ul><li>Flow control – scope is point-to-point; involves just sender and receiver. </li></ul>
  5. 5. Congestion Control, cont. <ul><li>Congestion Control is concerned with efficiently using a network at high load. </li></ul><ul><li>Techniques for detection and recovery are: </li></ul><ul><ul><li>Warning bit </li></ul></ul><ul><ul><li>Choke packets </li></ul></ul><ul><ul><li>Load shedding </li></ul></ul>
  6. 6. Warning Bit <ul><li>A special bit in the packet header is set by the router to warn the source when congestion is detected. </li></ul><ul><li>The bit is copied and piggy-backed on the ACK and sent to the sender. </li></ul><ul><li>The sender monitors the number of ACK packets it receives with the warning bit set and adjusts its transmission rate accordingly. </li></ul>
  7. 7. Choke Packets <ul><li>A more direct way of telling the source to slow down. </li></ul><ul><li>A choke packet is a control packet generated at a congested node and transmitted to restrict traffic flow. </li></ul><ul><li>The source, on receiving the choke packet must reduce its transmission rate by a certain percentage. </li></ul><ul><li>An example of a choke packet is the ICMP Source Quench Packet. </li></ul>
  8. 8. Hop-by-Hop Choke Packets <ul><li>Over long distances or at high speeds choke packets are not very effective. </li></ul><ul><li>A more efficient method is to send to choke packets hop-by-hop. </li></ul><ul><li>This requires each hop to reduce its transmission even before the choke packet arrive at the source. </li></ul>
  9. 9. Load Shedding <ul><li>When buffers become full, routers simply discard packets. </li></ul><ul><li>Which packet is chosen to be the victim depends on the application and on the error strategy used in the data link layer. </li></ul><ul><li>For a file transfer, for, e.g. cannot discard older packets since this will cause a gap in the received data. </li></ul><ul><li>For real-time voice or video it is probably better to </li></ul><ul><li>throw away old data and keep new packets. </li></ul><ul><li>Get the application to mark packets with discard priority. </li></ul>
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